A global assessment of salmon aquaculture impacts on wild salmonids

Abstract

Since the late 1980s, wild salmon catch and abundance have declined dramatically in the North Atlantic and in much of the northeastern Pacific south of Alaska. In these areas, there has been a concomitant increase in the production of farmed salmon. Previous studies have shown negative impacts on wild salmonids, but these results have been difficult to translate into predictions of change in wild population survival and abundance. We compared marine survival of salmonids in areas with salmon farming to adjacent areas without farms in Scotland, Ireland, Atlantic Canada, and Pacific Canada to estimate changes in marine survival concurrent with the growth of salmon aquaculture. Through a meta-analysis of existing data, we show a reduction in survival or abundance of Atlantic salmon; sea trout; and pink, chum, and coho salmon in association with increased production of farmed salmon. In many cases, these reductions in survival or abundance are greater than 50%. Meta-analytic estimates of the mean effect are significant and negative, suggesting that salmon farming has reduced survival of wild salmon and trout in many populations and countries.

Figure 1. Adult Returns of Wild Salmonids in Control (Black) and Exposed (Blue) Stocks, with Aquaculture Production (Red)

For plotting only, the returns to controls and exposed stocks have been separately summarized by a multiplicative model (log(Returns_{i ,y}) = a_i + d_y + e_{i ,y} ; variables are the same as in Equation 1). The mean returns across stocks for each year are shown. Note that left-hand axes are on a log scale. Only even year values are available for pink salmon prior to 1989. Irish salmon are not included because only marine survivals (not returns) are available.

Figure 2. Estimated Effects of Salmon Farming

All estimates are for Atlantic salmon unless otherwise noted. (A) Estimated percent change in survival of wild salmonids associated with salmon farming, per generation per tonne of farmed salmon production. (B) Estimated percent change in survival of wild salmonids associated with salmon farming, per generation, at the mean tonnage of farmed salmon harvested in each region, during the study period. The meta-analytic mean has been scaled to show mean reduction in survival when harvest of farmed salmon in the region is 15,000 t. (C and D) As for (A) and (B), but representing the change in returns to each stock (rather than survival). The bars represent 95% confidence intervals.